Wet processes for producing phosphoric acid by acid treating phosphate rock first appeared about a hundred years ago. Over the years numerous improvements and refinements in wet processing having appeared. Many of these are described in Phosphoric Acid edited by A. V. Slack (Marcel Dekker, Inc., New York, 1968). Wet process production of phosphoric acid appears to have been focused on the treatment of high grade phosphate rock and relatively little experience appears to be reported on the handling of low grade phosphate rock, i.e., phosphate rock having relatively low P.sub.2 0.sub.5 content of the order of 20 percent by weight and relatively high content of acid soluble materials such as iron oxide and aluminum oxide which are precursors of undesired impurities in the product acid.